Since the plasma CVD process has various advantages, for example, (i) deposited films can be formed at a low temperature from 200.degree. to 400.degree. C., (ii) heat resistance is not required for the substrate, etc., it has been applied so far for the preparation of silicon dioxide or silicon nitride as insulative films in semiconductor production processes, amorphous silicon (a-Si) films used for solar cells, as well as contact type image sensors or photosensitive drums, and diamond thin films.
Most of the conventional plasma CVD apparatus for conducting the plasma CVD process have been so adapted that radio frequency waves (RF) are charged between two opposing flat-parallel plate electrodes to generate plasmas. Since the RF plasma CVD apparatus is simple in structure, it has a merit that the size can be readily increased.
By the way, the conventional RF plasma CVD processes have the following drawbacks (1), (2) and (3): (1) Ionic sheaths are formed to each of the electrodes, a negative self-bias appears on the side of the cathode and ion species in plasmas are attracted to the cathode to moderate the incident impact of the ionic species to the anode to which the substrate is arranged. However, ionic species incident to the surface of the substrate are still present, and they are intaken into deposited films to increase the internal stress or density of defects, failing to obtain satisfactory deposited films,: (2) since the density of electrons is as low as from 10.sup.-8 to 10.sup.10, decomposing efficiency for starting material gases is not so great and the deposition rate is low,: and (3) since the electron temperature is as low as below 40 eV, it is difficult to decompose those starting material gases having high bonding energy such as halogenated silicon compounds.
In order to overcome the foregoing drawbacks (1), (2) and (3) in the plasma CVD process utilizing RF discharge, there have been developed plasma CVD apparatus using a microwave discharge system of applying electronic cyclotron resonance as disclosed in Japanese Patent Laid-Open Nos. Sho 56-155535 and Sho 59-3018. FIG. 8 illustrates one example of a schematic constitutional view for an ECR plasma CVD apparatus.
In FIG. 8, there are shown a plasma generating chamber 801 (having a cavity resonator structure), a magnetic field generating device 802, a microwave guide 803, a microwave introducing window 804, microwaves 805, plasma flow and diverging magnetic field 806, a deposition chamber 807, a specimen table 808 with heaters, a specimen substrate 809, a first gas introduction port 810, a second gas introduction port 811 and an exhaust system 813. In the ECR plasma CVD process, since electrons absorb energy of electromagnetic waves upon resonance under ECR conditions (requiring magnetic flux density of 875 gauss for microwave of 2.45 GHz), the electron temperature is high as up to 7 eV and electron density is also as high as 10.sup.11. Accordingly, a deposition rate of from 100 to 1000 .ANG./min can be obtained. Further, the incident ion energy is as small as upto 20 eV when compared with the parallel flat plate type plasma CVD process.
FIG. 9 shows the distribution of magnetic flux density within the plane of the substrate apart by 290 mm from the point when the magnetic flux density is 875 gauss under ECR conditions in a case where the diameter of the plasma generating chamber 801 is 170 mm in the ECR plasma CVD apparatus shown in FIG. 8. It can be seen that the range in which the magnetic flux density is uniform is a small region of about 80 mm.
In the ECR plasma CVD process, since the distribution of the magnetic flux density on the specimen substrate has an effect on the distribution of the thickness of the deposited film, it is necessary to make the magnetic flux film over a large area, that is, it requires solenoid coils of a great diameter. Accordingly, in an apparatus for depositing a thin film over a large area using the ECR plasma CVD process, since the solenoid coils of great diameter are necessary, it results in a drawback that the size and the weight of the apparatus are increased and the manufacturing cost for the apparatus is expensive.
As one of the methods for forming a deposited film over a large area by improving the foregoing drawbacks in the ECR plasma CVD apparatus, it may be considered to make the thickness of the deposited film uniform by moving the specimen substrate. In this method, however, if the diameter of the plasma generating chamber is set to 2a and the width of the specimen substrate to x, since it requires a length of about (2x-2a) for the width of the deposition chamber, the size of the deposition chamber is increased, which inevitably increases the size of the apparatus per se.
Another method has been proposed in Japanese Patent Laid-Open No. Sho 61-213377. One example for a schematic constitutional view for the plasma CVD apparatus in Japanese Patent Laid-Open No. Sho 61-213377 is as shown in FIG. 10. In FIG. 10, there are shown a plasma generating chamber 1001, a first magnetic field generating device 1002, a microwave guide 1003, a discharge tube 1004, microwave 1005, plasma 1006, a deposition chamber 1007, a rotary specimen table 1008, a specimen substrate 1009, a gas introduction port 1010, a second magnetic field generating device 1011, a third magnetic field generating device 1012 and an exhaust system 1013. In the plasma CVD apparatus shown in FIG. 10, if the area of the specimen substrate is increased, the size of the deposition chamber is necessarily increased to make the distance longer between the first and the second magnetic field generating devices, to thereby decrease the magnetic field component in parallel with the surface of the specimen substrate. Accordingly, it is expected that no uniform deposited film can be obtained. In addition, for obtaining a uniform deposited film, the magnetic field intensity generated by the third magnetic field generating device has to be strengthened which necessarily increases the size of the apparatus. In addition, deposited films are inevitably adhered to the wall surface of the deposition chamber attached with the third magnetic field generating device, which brings about a problem of reducing the utilizing efficiency of starting material gases and causing dusts due to peeling of the adhered films.